/snip
However, I do not see how you can absolve the airline whose systems of deciding who should have access to an airplane clearly failed.
Not to veer too far off course, but the employee in question worked for AA since 1988, presumably without incident.
If an employee goes rogue after over 30 years on the job due to a specific incident, I don't see how the airline could be deemed negligent in its hiring. Of course, if there was a history of malfeasance or inappropriate behavior and/or appropriate periodic reviews/background checks were not in place, that's a different story.